Pfizer
Animal Health US
Horse Health Management
Roundworms
Roundworms – Parascaris equorum
The roundworm, or
ascarid, is a prolific egg layer. Each female can lay from 100,000 to
200,000 eggs each day. The eggs pass out of the horse with the feces.
Infective larvae develop within the eggs, which are triple-coated and
are not affected by adverse weather conditions. Therefore, they remain
viable for up to 10 years. It is important to remember that fecal tests
do not detect migration of parasite larvae within the horse.
This picture shows the number of roundworms found in just one
foal. |
Ascarid Eggs
This is an ascarid egg, which may have been lying on the ground for
10 years. When it is ingested, it begins an amazing journey. The egg’s
coating is digested in the stomach. As the eggs reach the small intestine,
they hatch, and the larvae immediately penetrate the lining of the intestinal
tract, beginning a 30-day migration.
Liver With Ascarid
Larvae Damage
The eggs travel via the hepatic vein to the liver, where they eat their
way around the liver for seven to 10 days. The real damage takes place
here and in the lungs. Fortunately, the liver is a very resilient organ
and can regenerate itself. We seldom see any permanent damage to the
liver from ascarid larval migration.
Small Hemorrhages
From Ascarid Larvae Migration
The larvae then go to the lungs and continue their migration for 14–21
days, again eating their way around lung tissue. Damage done to the
lungs is a different story than that of the liver, because the lungs,
which heal by scarring, do not regenerate. This damage is permanent.
After the ascarids mature and are ready to complete migration, they
burrow from the blood side of the lung into the air side.
Normal Lung Tissue
This microscopic look at normal lung tissue shows empty air spaces
– the alveoli – where oxygen is exchanged for CO2.
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Lung Damage From
Ascarid Larvae Migration
This lung tissue has been damaged by ascarid larval migration.
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When migrating ascarid larvae are present, the immune system reacts
violently to the foreign protein and destroys the alveoli. Such damage
predisposes foals to pneumonia and may result in pulmonary hemorrhaging
in a horse that becomes an athlete. When a horse is just a few months
old, it has all the lung tissue it is ever going to have. Because lung
tissue heals by scarring, damage to these sensitive structures is permanent.
There will be less functional lung for the horse to use.
Horses whose lungs have been damaged by ascarid larval migration may
have to breathe harder and faster to meet their oxygen demands as they
develop and are asked to perform.
The worms then crawl from the alveoli into the bronchioles, to the bronchi,
and into the trachea. They cause enough irritation to elicit a cough,
so they are coughed to the back of the throat and re-swallowed as mature
larvae. As adults, they swim upstream in the small intestine, robbing
the horse of nutrition. These parasites have a very efficient migration.
When the larvae reach the small intestine for the second time, their
presence causes little consequence to the horse.
Ascarid Larval
Migration
Although horses may develop an immunity to ascarids, that immunity does
not prevent the migration and damage these parasites may cause.
Ascarid larval migration can lead to other diseases. It reduces overall
thriftiness in foals and can be related to foalhood pneumonia. Ascarid
larvae may have an immunosuppressive effect in the lungs, which means
they can reduce the immune system’s ability to respond to foreign invaders
like bacteria and viruses. Remember, fecal tests can’t detect these
migrating larvae.
Horses
Live in Contaminated Areas
Think for a moment about how these roundworm survival attributes could
affect your horses. The ascarids in your horses are still producing
hundreds of millions of eggs. The eggs are not affected by climatic
conditions, and they survive for up to 10 years. The only difference
between your horses and horses in a natural setting is that your horses
don’t migrate 25 miles a day away from these eggs. Instead, they are
contained in small grazing areas, which ensures that they ingest parasites.
Parasitism has become a chronic disease that horses are faced with daily.
Copyright
2004 - Pfizer, Inc.
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